Waste water entering a waste water treatment facility normally includes a substantial quantity of sand, grit and plastic, in addition to suspended settleable organic matter (sludge). One of the first steps in treating the waste water is to remove these non-organic solids so that the waste water can then be treated by oxidation and biodegradation techniques without the non-organic solids fouling or wearing out the mechanical components of the facility. Frequently the plant design requires that inorganic solids having a specific gravity of 2.65 and higher and a size of 65 mesh or larger should be removed before further treatment of waste water.
Although it is important that the non-organic solids be removed early in the treatment process, it is equally important that the organic solids such as sewage sludge not be removed at that point. This is because at that point the waste water has not been subjected to the oxidation and biodegradation processes that clean the water by consuming the sewage. Therefore, if the organic solids are removed along with the inorganic solids before the treatment process, then the removed solid will contain raw sewage which presents difficult problems of storage and disposal of a possibly hazardous waste.
Fortunately, the inorganic solids are generally distinguishable from the organic solids on the basis of their specific gravities. Inorganic solids such as sand and grit tend to be much denser than organic solids such as sludge. Therefore, inorganic solids tend to settle to the bottom of a fluid stream quicker than organic solids.
In most of the existing waste water treatment facilities, it is this difference in density between inorganic solids and organic solids that is used to remove the inorganic solids. Typically, the incoming waste water flows through an inorganic removal chamber that may be referred to as a "grit chamber" or "settling tank". The settling tank normally operates with a continuous flow through it, rather than as a batch at a time. Waste water flows into the chamber through an inlet and out through an outlet. At the bottom of the tank is generally a mechanism for collecting the material that settles to the bottom between the inlet and outlet. The collection mechanism is commonly a set of buckets on a continuous chain which scoop up the material and transport it and dump it into a collection point, or an airlift pump which pumps the grit from the bottom of the chamber to an outside collection point, or some other suitable mechanical or electromechanical system.
It can be appreciated that the removal efficiency of a settling tank is directly affected by the flow rate through the tank. With a high flow rate the retention time of the waste water in the tank is quite low, and so the solids have little time to settle to the bottom where they can be collected. In addition, a high flow rate tends to induce turbulence in the tank, which further impairs and slows the settling of solids. On the other hand, a low flow rate produces a high retention time for the waste water and also promotes laminar flow. The combination of a higher retention time and more laminar flow in low flow conditions as compared to high flow conditions, allows more solids to settle.
This relationship between flow rate and the efficiency of solids removal presents a problem in the design and operation of the settling tanks. It is desired that the flow rate through the settling tank be sufficiently low that inorganic solids are settled and removed, or at least the larger and denser inorganic solids are settled and removed. But as explained above, it is important that the flow rate not be so low that the organic solids are settled and removed. Therefore, unless the relationship between settling efficiency and flow rate is addressed in some manner, a settling tank will achieve the optimal balance between removal of inorganic solids and removal of organic solids at one flow rate, but will remove too little of the inorganic solids at higher flow rates and too much of the organic solids at lower flow rates.
The way this problem has typically been addressed is to induce a flow in the settling tank that is independent of the flow rate of waste water through the settling tank. This induced flow does not have much effect on the average retention times in the settling tank, but it does have an important effect on the turbulence of the settling tank. The settling efficiency then becomes more a function of the induced flow conditions than a function of the flow rate, thereby diminishing the effect of flow rate on settling efficiency. In addition, the induced flow can be made variable to respond to changing flow rates. At a high flow rate, which results in low retention times and high turbulence, the induced flow can be minimized so that the settling tank will collect the inorganic solids. At a low flow rate, which results in high retention times and low turbulence, the induced flow can be maximized so that the settling tank will not collect the organic solids. In other words, the induced flow mechanism varies the variables that determine settling efficiency (turbulence and water velocity) to offset changes in the other variable (retention times). The variances in the induced flow that are necessary to offset variances in the retention times can be determined by experimentation and by known principles of fluid dynamics based on the dimensions and configuration of the settling tank and the characteristics of the waste water.
The systems for inducing a flow in the settling tank are generally either those that induce a flow about a vertical axis or those that induce a flow about a horizontal axis. Those that induce a flow about a vertical axis commonly use an impeller mounted on a vertical shaft near the center of the settling tank, so that fluid circulates in one vertical direction through the impeller near the center of the tank and then in the opposite direction along the walls of the tank. Due to the action of the impeller, the fluid also tends to circulate about the impeller shaft, thereby resulting in a spiralling toroidal flow pattern as the fluid circulates about the shaft simultaneously with its vertical movement. While such a "vertical shaft mixer" design can be effective in theory, tanks with such designs tend to be quite inefficient because of the asymmetric flows as seen in plan view, and turbulence caused by the conservation of angular momentum, i.e. the Coriolis effect.
The other type of system for inducing a flow in the settling tank, in which the flow is about a horizontal axis, typically relies upon the injection of compressed air from a line of jets positioned horizontally in the settling tank. The horizontal position of the line of jets is generally offset from the horizontal centerline of the settling tank, so that the rising air bubbles in the fluid drive the fluid upward on the side of the tank having the jets, across the top of the tank toward the wall opposite the jets, down the wall opposite the jets and across the bottom of the tank back to the wall with the jets. At the same time, the fluid is flowing through the tank perpendicular to the direction of the induced flow, so that the overall result is a spiral flow about the longitudinal axis of the tank. A serious drawback to the use of compressed air jets to induce flow in the settling tank is that the degree of induced flow cannot be modulated closely as a function of flow rates. The induced flow tends to be either "on" or "off" but with no reliable and precise way of adjusting it.
It can be appreciated that there is a need for a system to produce an induced flow in a settling tank in a way that is reliable, simple and adjustable. Preferably such a system would be adaptable for use with settling tanks that are currently in existence without the need for extensive design changes.